Vapour quenching is a known process by which alloys having metastable supersaturated solid solutions can be produced. The process comprises evaporation of alloy constituents under vacuum from individual or combined sources to produce a flux of vapours and condensation of the vapours upon a temperature-controlled collector. The collector deposit may be mechanically worked in situ to consolidate the deposit and thus reduce any tendency towards porosity, or may be hot isostatic pressed after removal from the collector. Not all alloys require this subsequent consolidation step. Some are satisfactory in the as-deposited condition, depending on composition, deposition conditions and intended utilisation. A description of this process as applied to aluminium-based alloys is given in two articles by Bickerdike et al. in the International Journal of Rapid Solidification, 1985, vol. 1 pp. 305-325; and 1986, vol. 2, pp. 1-19.
Titanium alloys are well established in the field of aerospace structures and in aero engine applications because they possess a high strength-to-weight ratio at high temperatures. In these known alloys, titanium is commonly alloyed with one or more of the following elements in the indicated proportions by weight:
Al (up to 8%); Sn (up to 11%); Zr (up to 11%); Mo (up to 15%); V (up to 13%); Si (up to 0.5%), and Cr (up to 11%).
Commercial purity titanium may contain the following elements in addition to those strengthening ingredients found in titanium alloys:
O.sub.2 (up to 0.32%); N.sub.2 (up to 0.006%); C (up to 0.02%); H.sub.2 (0.002 to 0.004%), and Fe (up to 0.05%), together with trace quantities of other metals (0.05% or less). Proportions are again indicated by weight. Such elements are normally considered to be impurities but oxygen at least contributes to the hardness of the titanium material and is sometimes considered as a minor but significant alloying ingredient.
The present invention provides new alloys based on titanium incorporating an element from the group consisting of magnesium, calcium and lithium. Little published data is available regarding these three binary systems and it is virtually impossible to incorporate any of these into a titanium alloy made by mixing in the melt because all three elements boil at a temperature below the melting point of titanium. Certain limited data is available for these systems from the evidence of diffusion couples which reveals that all three of these elements have negligible equilibrium solid solubility in titanium (less than 0.5% by weight) and which further indicates that none of them form compounds when present in titanium.
We have discovered that it is possible to make binary alloys of titanium with magnesium, which alloys retain the magnesium constituent in solid solution within the titanium in quantities far greater than the equilibrium solid solubility limit, by producing these alloys using the vapour quenching route. Furthermore, we have found that these magnesium-containing alloys show an appreciable age hardening response which was previously unknown. It is believed that these findings will read across to the closely related elements calcium and lithium which can also be produced as binary alloys with titanium by means of vapour quenching, and that the findings will be valid to a greater or lesser extent when any of these three elements is introduced into a vapour-quenched titanium alloy containing any of the known alloying additions for titanium alloys.
The elements magnesium, calcium and lithium each have densities much lower than that of titanium. Since existing data indicates that none of these elements form compounds when present in binary titanium alloys, this leads to the conclusion that the density of the binary alloy may be predicted reliably by the rule of mixtures. We have found that the rule of mixtures method holds good for vapour-quenched alloys of the titanium-magnesium system at least. Accordingly, those of the alloys claimed hereinafter which incorporate say 2% or more by weight of any of the above elements will exhibit appreciable density reductions which could make them attractive as replacements for present titanium alloys in applications where weight savings are important.
There have been prior reports (for example, in the article by Suryanarayana and Froes, J. Mater. Res. 9 [1990] 1880) of titanium-magnesium alloys produced by another route, namely mechanical alloying. This is a process involving the ball milling under vacuum or an inert atmosphere of separate alloy constituents in powder or similar form to produce small pieces of alloy by agglomeration.
In the reported experiments, the starting material comprised a 9% magnesium/91% titanium mixture in proportions by weight and this was worked in the ball mill for several successive periods between which samples of the agglomerated products were withdrawn and analysed.
The above-mentioned authors concluded from their experiments that they were successful in introducing into solid solution within the mechanically alloyed pieces all the 3% magnesium which analysis revealed to be contained in the material. The evidence advanced to support this conclusion relies on a conventional lattice parameter measurement, but the quoted measurements indicate that the lattice parameters decreased rather than increased with longer periods of ball milling. However, a large increase in lattice parameter was observed when the particulate product was annealed at high temperature.
The atomic radius of magnesium is greater than that of titanium, which means that a solid solution of magnesium in titanium would be expected to lead to an increase in lattice parameter rather than to a decrease. Consequently, the observed decrease in lattice parameter points to some other conclusion than the magnesium being present in solid solution.
By its very nature, the process of mechanical alloying is a long one. With a starting component as reactive with oxygen as magnesium, it will be difficult to avoid oxygen contamination even though the milling is conducted under vacuum or in an inert atmosphere, not least because of the likelihood of oxygen being present in the starting materials. Moreover oxygen is likely to be picked up during the annealing treatment applied to such materials.
The observed lattice parameter increase upon annealing would seem to be more consistent with the conclusion that much of the magnesium in the agglomerated material is present as free magnesium or as an oxide. It does not appear to support the claim that the measured 3% of magnesium in the annealed material is present in solid solution within the titanium.